The influence of the presence of an inflammatory rheumatic disease and its treatment on the severity of and immune response to (viral) infections is not clear. The emergence and pandemic of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) provides the opportunity to assess these influences on COVID-19 incidence, its clinical severity, and the antibody response, compared to a control population. The primary objective will be to compare the disease severity of COVID-19 between patients with a systemic autoimmune disease and a control group. Disease severity is defined as the (unplanned) hospital admission rate of participants that are both IgM- or IgG-SARS-CoV-2 antibody positive and symptomatic. Symptomatic is defined as symptoms or signs of nasopharyngitis, cough, dyspnea, fever, or any other symptom or sign that may be associated with a viral infection, as assessed by the patient. Unplanned refers to the fact that elective hospital admissions (e.g., for planned surgery) are excluded.

One of the secondary objectives is to study the following differences between patients and controls, and subsequently, between conventional DMARD (including glucocorticoid) use and biological use, will be studied:

- The cumulative (6-month) incidence of IgM or IgG antibodies against SARS-CoV-2;

We will also investigate whether physicians and/or patients decide to adjust the use or dose of DMARDs due to the SARS-CoV-2 pandemic, and how these potential adjustments influence disease activity.

This is a prospective observational cohort study with a follow-up of 6 months. The baseline measurement will consist of a digital survey covering demographic variables, specific health-related topics, and COVID-19 relevant questions. Participants will be asked to fill in the same survey after 1-3 and 4-6 months of follow-up. In addition, during follow-up, blood will be drawn at Reade two times within approximately one week of completion of the online surveys. The study population will consist of participants with a systemic autoimmune disease. Patients will be recruited at all locations of the Amsterdam Rheumatology & immunology Center (ARC) (i.e., Reade and Amsterdam UMC locations AMC and VUmc). Each patient will be asked to provide a healthy control without a systemic autoimmune disease from his/her social group or household to ensure that there is a comparable chance of exposure to SARS-CoV-2 in the control group. All participants will be at least 18 years old. In total, we expect to include 8000 subjects. All subjects will be asked to complete the online survey three times. This survey covers several topics, among which respiratory illnesses (including hospital and or ICU admittance), rheumatic disease activity and medication use. Demographic data will also be collected at baseline. In addition, blood will be drawn during two visits at Reade after 1-3 and 4-6 months of follow up.